FCPX and Motion both come with a specific set of "application installed" fonts, like Junegull, Flatbush, Gaz Transport, etc. (there are 73 font files in this location). You can copy them out of FCPX to install in your system with Font Book for use in all other applications installed on your Mac.
First: Go into Applications and right click on Final Cut Pro. Select Show Package Contents.
In the folder that opens, navigate to:
Contents / Frameworks / Flexo.framework / Versions / A / Resources / Fonts
Select all the fonts and OPTION-DRAG them to another location (have a Finder folder prepared to receive them in advance). [DO NOT move those fonts from their original location!]
Open Font Book.
[Optional] At the bottom of the left column, click on the plus icon and create a New Collection. Rename the collection FCPX Collection (or similar). Drag all the fonts from the Finder folder you prepared into the next column to add them to the collection. Does this step matter? No - you may drag all the fonts into the All Fonts collection. However, in Motion, you can select individual font collections to narrow selection down to only those fonts. It helps to have the FCPX fonts in one location, so to speak. Otherwise, it won't make any difference to the rest of your installed applications what collection they are in.